1. Field of the Invention
The present invention relates to a power steering apparatus for use in association with an electric motor so as to perform the feedback control of a motor-driving current, wherein the steering apparatus includes a motor driving circuit which is driven in response to PWM (pulse width modulation) wave signals.
2. Description of the Background Art
FIG. 1 is a block diagram showing a motor driving circuit and a motor-driving current detecting circuit used in a known power steering apparatus:
The illustrated power steering apparatus includes a motor driving circuit 13 which includes bridge-connected transistors (switching elements) T1 to T4, and a motor M connected in such a manner as to connect two junctions, that is, the junctions between the transistors T1 and T3 and between the transistors T2 and T4. The pair of transistors T1 and T4, and the pair of transistors T2 and T3 are respectively connected in series through the motor M. Each pair of transistors T1-T4 and T2-T3 are driven in response to the PWM wave signals PWM.sub.CCW and PWM.sub.CW, from a PWM wave generator (not shown) received at the respective gates. The PWM wave signals PWM.sub.CCW and PWM.sub.CW are mutually complementary in pulse width as shown in FIG. 2 wherein each has a duty ratio of 50%, thereby allowing no motor-driving current to flow.
The transistors T1 and T2 are located toward a power source P for the motor M, and the transistors T3 and T4 are located toward the earth. The sources of the transistors T1 and T2 are connected to the power source P, and the drains of the transistors T3 and T4 are connected to one terminal of a resistance Rs for detecting a motor-driving current, the other terminal of which is grounded. The resistance Rs (hereinafter "detecting resistance Rs") has a source-side terminal connected to a non-invertible input terminal of a differential amplifier 10, and an earth-side terminal connected to an invertible input terminal thereof. In this way, the voltage developed across the detecting resistance Rs generated by a motor-driving current are inputted to the differential amplifier 10.
In the motor driving circuit 13 and the differential amplifier 10, in a case where the duty ratio of the PWM wave signal PWM.sub.CW is larger than that of the PWM wave signal PWM.sub.CCW, if the PWM wave signal PWM.sub.CW is on, and the PWM wave signal PWM.sub.CCW is off, the transistors T2 and T3 are on, and the transistors T1 and T4 are off, thereby enabling the motor-driving current to flow as indicated in the full line in FIG. 1. At this stage, the input voltage V.sub.+ (in the case of the full line) at the non-invertible input terminal of the differential amplifier 10 becomes positive as shown in FIG. 3 (wherein the invertible input terminal is grounded), thereby enabling the differential amplifier 10 to output a positive voltage.
When the PWM wave signal PWM.sub.CW is off and the PWM wave signal PWM.sub.CCW is on, the transistors T2 and T3 are off and the transistors T1 and T4 are on. Because of the inductance of the motor M, a voltage is induced in such a direction as to prevent the motor-driving current from abruptly changing in the motor M. The induced voltage progressively becomes stronger than the voltage at the power source P, thereby enabling the motor-driving current to flow through the transistors T1 and T4 as indicated in the dotted line in FIG. 1.
Within the motor M the motor-driving current flows in the same direction (from right to left in FIG. 1) as the direction in which it flows when the PWM wave signal PWM.sub.CW is on, but since it flows through the transistors T1 and T4, it flows through the detecting resistance Rs in an opposite direction (from down to up in FIG. 1) to the direction in which it flows when the PWM wave signal PWM.sub.CW is on. At this stage, the input voltage V.sub.- (in the case of the dotted line) at the invertible input terminal of the differential amplifier 10 becomes higher than the input voltage V.sub.+ at the non-invertible terminal thereof, thereby enabling the differential amplifier 10 to output a negative voltage as shown in FIG. 3.
Under the known arrangement referred to above, a different value (polarity is opposite) from that of a motor-driving current within the motor M is detected. As a result, if the output of the differential amplifier 10 is used to perform the feedback control of the motor-driving current as it is, no smooth steering touch or feeling cannot be achieved. In order to use the output of the differential amplifier 10 to correctly perform the feedback control of the motor-driving current, an extra circuit is required to judge the direction of the flow of the motor-driving current.